TOM WALKER WESTMINSTER ABBEY

Tom Walker singing at Westminster Abbey with the lovely Duchess of Cambridge playing the piano. 

This article says it was pre-recorded the day prior to the service. 

https://www.dailymail.co.uk/femail/article-10343913/Kate-Middleton-leaves-viewers-speechless-touching-piano-performance.html

https://www.youtube.com/watch?v=yw95R0Vl4_c

I have a sweet memory of singing solo at the abbey at Christmas time. Our school was close-by.